Studies on the in vitro transcription and translation of the lac operon

Prokaryotic gene expression in vitro: transcription-translation coupled systems

Transcription-translation coupled systems have been developed to study prokaryotic gene expression. Several types of expression system have been described. The original system consists of a crude unfractionated Escherichia coli extract, which supports protein synthesis directed by a template DNA. Control of gene expression at the transcriptional stage has been studied using this unfractionated system. In this respect, two examples of particular interest, lactose and tryptophan operons, are described. Other systems are either partially reconstituted or highly defined, containing up to 30 purified factors necessary for transcription (RNA polymerase) and translation (aminoacyl-tRNA synthetases, initiation, elongation and release factors). Additional differences between the various systems relate to the analysis of the gene products. Whereas most methods involve analysis of the totally synthesized protein, a particular system implies the formation of only the N-terminal di- or tripeptide of the gene product. Reconstituted systems have proved useful in studies on transcriptional, e.g., discovery and role of L factor, as well as translational regulation of gene expression, e.g., autogenous control of ribosomal protein synthesis.

Effect of guanosine 5'-diphosphate 3'-diphosphate and related nucleoside polyphosphates on induction of tryptophanase and beta-galactosidase in permeabilized cells of Escherichia coli

Exogenous addition of guanosine and adenosine 5'-(mono, di and tri) phosphate 3'-diphosphates (pppGpp, ppGpp, pGpp, pppApp, ppApp and pApp) stimulated the synthesis of tryptophanase and beta-galactosidase in permeabilized cells of Escherichia coli. From the results obtained with ppGpp and pppApp, this effect appeared to be at a transcriptional level and depended greatly on the growth condition; the largest effect was observed in cells under shiftdown or grown on poor enrgy source. ppGpp and pppApp, unlike cyclic AMP, did not act to overcome the inhibition of enzyme induction by glucose, but in combination with cyclic AMP caused a synergistic stimulation effect. In the shiftdown cells, ppGpp and pppApp gave 30% or more stimulation effect on tryptophanase induction while cyclic AMP did not stimulate induction. There was therefore a pronounced difference between cyclic AMP and ppGpp or pppApp in stimulatory function.

Effect of guanosine tetraphosphate on in vitro protein synthesis directed by E1 and E3 colicinogenic factors

The in vitro synthesis of colicin E3 was found to be stimulated by guanosine 5'-diphosphate 3'-diphosphate (ppGpp), while that of several other ColE3 plasmid-specific proteins was reduced in the presence of this nucleotide. The ColE1 plasmid-directed in vitro synthesis of colicin E1 was also found to be stimulated by ppGpp.

DNA-directed in vitro synthesis of elongation factor Tu

The in vitro synthesis of EF-Tu was studied using the transducing phage lambdarifd18 DNA as template. The EF-Tu synthesized was identified by its immunological properties, gel analysis, and its ability to interact with GDP and EF-Ts. Two other unidentified polypeptides were also precipitated from the incubation mixture by EF-Tu antiserum. The synthesis of total EF-Tu immunoprecipitable material was depressed about one-half by guanosine-5'-diphosphate-3'-diphosphate which was shown to inhibit the transcription process.

DNA dependent synthesis of protein L12 from escherichia coli ribosomes, in vitro

The in vitro synthesis of ribosomal protein L12 has been obtained in a coupled system with DNA extracted from the transducing phage lambdarifd18 as template. In addition, a second protein (molecular weight of 16,000) with immunological, chemical, and ribosome-binding characteristics similar to L12 is formed in this in vitro system. The synthesis of both proteins is depressed by one-half when guanosine-5'-diphosphate-3'-diphosphate is added to the reaction mixture.

Effects of methylation of the beta-galactosidase genome upon in vitro synthesis of beta-galactosidase

A template DNA from phage lambdah80dlacp5 coding for the in vitro synthesis of beta-galactosidase was used to study the effect of DNA methylation by the alkylating agent, dimethyl sulfate (DMS). Increasing the levels of DMS up to 50 mM concentration in the incubation medium led to an increase of DNA methylation. When incubated for 10 min at 37 degrees C, 3-4% Of nucleotides were methylated. The increase was linear to about 0.6% nucleotide methylation level. A higher yield was obtained at 37 degrees C incubation temperature than at 20 degrees C. Methylation of lambdah80dlacp5 DNA alone without methylation of other factors in the incubation mixture caused inhibition of the synthesis of beta-galactosidase in vitro. Increasing levels of DNA methylation caused greater inhibition of the newly synthesized enzyme activity. Total protein and RNA synthesis was inhibited by the methylated DNA to a much lesser extent than the inhibition of enzyme activity. When the level of nucleotide methylation was 0.74%, only 2% of enzyme activity remained, but total protein and RNA synthetic activities were found to be 72% and 44%, respectively.

Effects of guanosine tetraphosphate on cell-free synthesis of Escherichia coli ribosomal RNA and other gene products

A cell-free system derived from E. coli is described in which mature-sized 16S and 23S ribosomal RNAs (rRNA) are synthesized at a high relative rate, comprising 17-25% of the total transcription. The addition of guanosine tetraphosphate (ppGpp) to this system results in up to a 5-fold selective inhibition of rRNA accumulation. This effect is exerted at the level of synthesis rather than degradation. It is concluded that ppGpp, which is produced in large amounts by E. coli during amino-acid deprivation, could mediate the decrease in rRNA synthesis that accompanies such deprivation. The expression of other genes has also been investigated. No selective reduction of transfer RNA synthesis by ppGpp is observed. The trp and lac operons are found to be stimulated at the transcriptional level by the presence of this nucleotide. It is hypothesized that ppGpp interacts with the RNA polymerase in such a manner as to alter the affinity of the enzyme for promoters in an operon-specific fashion.